The pandemic influenza A(H1N1) virus has been virtually the only influenza virus circulating in the United States since it was first identified in April 2009, according to a report from Quest Diagnostics.
In an analysis of 195,000 lab tests performed at five of the company's laboratories, 99% of samples testing positive for influenza A were identified as H1N1, according to the report, titled “H1N1 Testing in America: H1N1 the Dominant Flu of 2009–2010.” “Our laboratory testing data also show an absence of influenza B viruses,” it noted. “These findings suggest the H1N1 virus has 'crowded out' other flu viruses.”
The data confirm that two waves of H1N1 infection have occurred, with no evidence of a third wave, the report's authors explained. The first wave, beginning in April or May 2009, ended in mid-August 2009. The second wave started in late August/early September 2009, and peaked in late October 2009. “Our data suggest that the return of children to school in the fall was likely the trigger for a second wave of H1N1 infection,” the report's authors wrote. “This trend—where children are the first to suffer from a new influenza virus that then spreads to other age groups—is consistent with the behavior of prior flu viruses.”
The report showed that H1N1 positivity was highest among children aged 10–14 years during both waves. Among that age group, the rate of H1N1-positive tests was 83% and 82% in the first and second waves, respectively. That compares with rates of 76% and 78%, respectively, in the 5- to 9-year age group. Among adults aged 25–49 years, the rates were 46% and 50%, respectively.
Since the end of the second wave, older children continue to have the highest positivity rates, compared with the other age groups, according to the report. In the 4 weeks ending April 15, 2010, 26% of tests were positive in the 10- to 14-year age group, compared with 18% among younger children and 13% among adults.
When analyzed by geographic region, positivity rates during the same 4-week period were highest (26%) in the southeastern United States (Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, and Tennessee). In the central southern states (Arkansas, Louisiana, New Mexico, Oklahoma, and Texas), the rate was 22%. Those rates are in contrast to a combined 6% rate in the rest of the country.
The full report is available at www.QuestDiagnostics.com/HealthTrends
The H1N1 virus has “crowded out” other influenza A and B viruses.
Source Courtesy CDC